During the production of injection-molded parts, especially injection-molded parts made of plastics, there is often a conflict in aims between the surface properties of the plastic, and hence its appearance, and the other properties, which influence the technical applications of the injection-molded part. A familiar example here is plastic polypropylene (PP). The surface finish of injection-molded parts made of PP is optically very advantageous if, as is often the case, a matte, non-reflecting surface is required. However, the scratch resistance of polypropylene is limited, which naturally reduces the application potential. For many applications, moreover, it is important to use plastics with higher scratch resistance. A typical example here is plastic acrylonitrile butadiene styrene (ABS), which for its part, however, has unfavourable surface properties in so far as the accuracy of surface-detail reproduction during the molding of injection-molded parts is concerned. If, using the plastic ABS, one wishes to obtain a surface impression that is matte and not highly glossy, or a surface without a large number of glossy spots, it is necessary to create a degree of roughness at the surface of the plastic so that incident light is scattered diffusely and not reflected as if by a mirror.
The production parameters that determine the surface finish of an injection-molded part are pressure, temperature of the injected material, and the time taken for a surface layer of the material to solidify. The material can, in principle, be any material that has the property of melting, or at least of converting to a flowable state, on heating, and of solidifying again—without the material being destroyed—on subsequent cooling. Many materials have this property, including thermoplastics, glass, elastomers and, to a certain extent, thermoset plastics if these are formed—for example by molecular cross-linking—when they are heated and then cooled again during the molding process.
The injection-molding process comprises the steps of introducing the molten material under pressure into a temperature-controlled or cooled molding tool and cooling the material until it has solidified sufficiently to permit removal of the injection-molded part. The main controllable variables available for influencing the surface finish of the injection-molded part are thus the pressure, the temperature of the molten material when it is injected into the mold, and the molding-tool temperature, which is controlled by way of active cooling. Of these parameters, the pressure can only be controlled within narrow limits, since a minimum pressure is necessary to ensure that the mold is filled completely and that the molding process does not take too long. The maximum pressure is limited by the maximum force that may be exerted on the mold. The temperature of the material when in the molten state is predetermined by the properties of the material. In the case of material blends, an insufficiently high temperature may result in separations, inadequate flow and too great a viscosity. Very high temperatures may destroy the material itself.
According to the prior art, it has hitherto been possible to work the surfaces of a mold such that they exhibit a degree of roughness that would suffice, for example, to scatter light diffusely in the manner described above. This effect can be obtained, for example, with spark-eroded or photo-etched surfaces. However, in the endeavor to transfer this very fine surface texture to the injection-molded part, differences in gloss level result because the fine texture is not reproduced uniformly in all areas and high-gloss spots are formed. The main reason for this is premature “freezing” of the molding material before it has penetrated into the fine surface texture of the mold. Such differences in gloss may also be only of localized occurrence if non-uniform cooling means that reproduction of the surface details is only prevented in certain areas, or that the injection-molded part shrinks more quickly in certain areas.
Gloss, hue and lightness are important factors for determining optical properties, in particular of articles made of plastics. Gloss is usually subdivided into the following categories: high-gloss, glossy, semi-gloss/silky, semi-matte/satin-matte, matte and dull matte. The individual categories are measured using a gloss meter and selecting different measuring angles. According to the German standard DIN 55 945 (Terms and definitions for coating materials), gloss is a sensation that is caused by the reflection of light beams at the surface of a coating and is perceived by the human eye. In the case of a smooth, matte surface, the incident light is scattered and reflected uniformly in all directions. By contrast, a smooth, high-gloss surface reflects the visible light in one direction, without any scatter, such that the angles of incidence and reflection are always the same. To make gloss measurements that compare well with the visual impression conveyed at different gloss levels, it is necessary, when using a gloss meter, to vary the angle of incidence of the visible light. In the transition range between glossy and matte, the measurement is performed at 60°. As was explained above, however, there are many applications in which matte surfaces are preferred, especially in the case of injection-molded plastics products.
In practice, the color value L is usually measured using the L*, A*, a*, b* system developed by Judd and Hunter and standardized in 1976 (DIN 6174, CIE LAB 1976). The L*-value represents the luminance, that is to say the lightness (0 to 100), and the two color channels a* and b* represent the values from green to red and blue to yellow respectively (from −127 to 128 in each case).
As explained, the surface-relevant factors gloss level and color value L*, which are important in particular for articles made of plastics, can be measured with appropriate equipment. The values obtained are free of subjective sensations of the human eye and can therefore be compared with each other on an objective basis.
Relatively expensive and time-consuming secondary finishing—for example a paint finish for certain plastic articles—is consequently necessary if, on account of other requirements, plastics have to be used that are apt to behave in the manner described above. A uniform, matte surface could be obtained, however, if the surface of the plastic were an exact reproduction, including the finest details, of the surface of the mold.
The method of obtaining a more accurate reproduction of the mold surface by selecting a higher mold temperature has the disadvantage that it necessitates a longer cooling time, hence prolonging the molding-cycle time. In addition, sink-mark formation is more pronounced because the plastic's ability to relax is increased.
It is also prior art to provide the surfaces of injection molds for plastics with thin coatings of hard metals, especially metal nitride or metal carbide compounds. The plastic's adhesive properties and hence the amount of force needed for removing the plastic from the injection mold are thereby reduced. On account of their excellent resistance to mechanical abrasion, these coatings also serve to reduce wear on the mold surface. It is desirable, however, that these coatings be applied as thinly as possible so as not to impair the dimensional accuracy of the injection mold, or to avoid having to make allowance early on, during production of the injection mold, for later dimensional changes caused by the coating.